1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a metal-doped transition-metal hexacyanoferrate (TMHCF) battery electrode, and associated fabrication processes.
2. Description of the Related Art
A battery is an electrochemical cell through which chemical energy and electric energy can be converted back and forth. The energy density of a battery is determined by its voltage and charge capacity. Lithium has the most negative potential of −3.04 V vs. H2/H+, and has the highest gravimetric capacity of 3860 milliamp-hours per gram (mAh/g). Due to their high energy densities, lithium-ion batteries have led the portable electronics revolution. However, the high cost of lithium metal renders doubtful the commercialization of lithium batteries as large scale energy storage devices. Further, the demand for lithium and its reserve as a mineral have raised the need to build other types metal-ion batteries as an alternative.
Lithium-ion (Li-ion) batteries employ lithium storage compounds as the positive (cathode) and negative (anode) electrode materials. As a battery is cycled, lithium ions (Li+) are exchanged between the positive and negative electrodes. Li-ion batteries have been referred to as rocking chair batteries because the lithium ions “rock” back and forth between the positive and negative electrodes as the cells are charged and discharged. The positive electrode (cathode) material is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material having a tunneled structure, such as lithium manganese oxide (LiMn2O4), on an aluminum current collector. The negative electrode (anode) material is typically a graphitic carbon, also a layered material, on a copper current collector. In the charge-discharge process, lithium ions are inserted into, or extracted from interstitial spaces of the active materials.
Similar to the lithium-ion batteries, metal-ion batteries use the metal-ion host compounds as their electrode materials in which metal-ions can move easily and reversibly. As for a Li+-ion, it has one of the smallest radii of all metal ions and is compatible with the interstitial spaces of many materials, such as the layered LiCoO2, olivine-structured LiFePO4, spinel-structured LiMn2O4, and so on. Other metal ions, such as Na+, K+, Mg2+, Al3+, Zn2+, etc., with large sizes, severely distort Li-based intercalation compounds and ruin their structures in several charge/discharge cycles. Therefore, new materials with large interstitial spaces would have to be used to host such metal-ions in a metal-ion battery.
FIG. 1 is a diagram depicting the crystal structure of a transition metal hexacyanoferrate (TMHCF) in the form of AxM1M2(CN)6 (prior art). TMHCF with large interstitial spaces has been investigated as a cathode material for rechargeable lithium-ion batteries [1,2], sodium-ion batteries [3,4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates ((Cu,Ni)-HCFs) exhibited a very good cycling life with 83% capacity retained after 40,000 cycles at a charge/discharge rate of 17 C [6-8]. However, the materials demonstrated low capacities and energy densities because: (1) just one sodium-ion can be inserted/extracted into/from each Cu-HCF or Ni-HCF molecule, and (2) these TMHCF electrodes must be operated below 1.23 V due to the water electrochemical window. To correct these shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) have been used as cathode materials in non-aqueous electrolyte [9, 10]. Assembled with a sodium-metal anode, the Mn-HCF and Fe-HCF electrodes cycled between 2.0 V and 4.2 V and delivered a capacity of about 110 mAh/g.
TMHCF has two main disadvantages as an electrode material in rechargeable batteries with a non-aqueous electrolyte. One is that water molecules reside in the larger interstitial spaces of TMHCF. The other is its low electronic conductivity.
In TMHCF, interstitial water demonstrates a complicated behavior. On one hand, interstitial water supports the TMHCF framework and stabilizes its structure. On the other hand, when TMHCF is used in rechargeable batteries with a non-aqueous electrolyte, the interstitial water promotes adverse effects on TMHCF performance. In general, non-aqueous electrolyte rechargeable batteries work at high voltage ranges that are beyond the water decomposition voltage. In TMHCF batteries, the interstitial water decomposes at the high operation voltage. The disappearance of interstitial water makes the TMHCF framework unstable, which shortens the capacity retention of TMHCF electrodes. The interstitial spaces of the TMHCF occupied by water molecules reduce the concentration of movable ions in the interstitial spaces, which leads to the small capacity of TMHCF electrodes during charge/discharge. For example, the nominal formula of sodium Mn-HCF is Na2MnFe(CN)6. But due to the interstitial water molecules, just 1.32 sodium-ions exist in one Mn-HCF molecule. Its maximum capacity was measured to 112 mAh/g [9].
The general formula of TMHCF can be expressed as AxMyFez(CN)n.mH2O, in which “A” is alkali-ion or alkaline-ion, and “M” indicates one of several transition metals. During charge/discharge, the following reaction takes place:AxMyFez(CN)n.mH2OxAa++[MyFez(CN)n.mH2O]xa−+xae−.
In terms of the reaction, the performance of the TMHCF electrode is determined by how fast A-ions and electrons transport in the TMHCF framework. However, electron transfer is difficult along the Fe—C≡N-M structure. In other words, TMHCF has a low electronic conductivity. Therefore, TMHCF electrodes exhibit poor performance when a high charge/discharge current is exerted onto the batteries.
It would be advantageous if TMHCF could be doped with metal ions to improve its performance as a rechargeable battery electrode, to increase capacity and lengthening cycling life.    [1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.    [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron cyanide complex as positive electrode of lithium secondary battery, Journal of Power Sources, 79 (1999) 215-219.    [3]Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Prussian blue: a new framework for sodium batteries, Chemistry Communication, 48 (2012) 6544-6546.    [4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, J. B. Goodenough, A superior low-cost cathode for a Na-ion battery, Angew. Chem. Int. Ed., 52 (2013) 1964-1967.    [5] A. Eftekhari, Potassium secondary cell based on Prussian blue cathode, J. Power Sources, 126 (2004) 221-228.    [6] C. D. Wessells, R. A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2 (2011) 550.    [7]C. D. Wessells, S. V. Peddada, R. A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Letters, 11 (2011) 5421-5425.    [8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, Y. Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode, J. Electrochem. Soc., 159 (2012) A98-A103.    [9] T. Matsuda, M. Takachi, Y. Moritomo, A sodium manganese ferrocyanide thin film for Na-ion batteries, Chemical Communications, DOI: 10.1039/C3CC38839E.    [10]S.-H. Yu, M. Shokouhimehr, T. Hyeon, Y.-E. Sung, Iron hexacyanoferrate nanoparticles as cathode materials for lithium and sodium rechargeable batteries, ECS Electrochemistry Letters, 2 (2013) A39-A41.